Forecasting Typhoon Chaba's (2004) intensity change using a coupled atmosphere-ocean-wave model

Recorded 10 typhoons have destroyed many lives of coastal residents on the Japan islands in the 2004 Northwest Pacific typhoon season. To reduce disaster risks, an early warning system which can provide highly accurate forecasts of the track and intensity of an approaching storm, must be implemented and refined. Although rapid developments of meteorological, remote sensing and computing technologies make it possible to predict the track of a tropical cyclone with high accuracy, it would be still difficult to evaluate the storm intensity even in short-term forecasts because of the following reasons: 1) still lack of spatial resolution in current models, 2) the inadequate parameterization of key air-sea interaction processes, 3) the inconsistency of typhoon initialization methods which use a tropical cyclone bogussing scheme.

The objectives of this study are to develop a high-resolution atmosphere-ocean-wave coupled model for tropical cyclone forecasts over the Northwest Pacific Ocean, and to quantify the impact of complex sea surface processes on Typhoon Chaba's (2004) intensity, as a case study of a hazardous event which affected the coastal areas of the Japan islands in August 2004.

A coupled atmosphere-ocean-wave model to be developed in this study is based on three existing models: MM5 (Dudhia, 1993) with an original typhoon bogussing scheme, CCM (Murakami et al., 2004), and SWAN (Booij et al., 1997). The coupled model simultaneously represents both atmospheric and ocean processes, and interactively exchanges information at the interface between the three model components, at every 10 minutes. The forecasting period is 48 hours from 1200 UTC 27 through 1200 UTC 29 August in 2004, when Typhoon Chaba gradually got weaker before landfall.

We have conducted two types of sensitivity experiments: 1) using the coupled model developed in this study (CPLD), and 2) using a single MM5 model (SNGL). There is a large difference in the typhoon evolution between these two models. The typhoon intensity in CPLD varies from 930 hPa to 950 hPa during the 48 hours integrations, similar to that of the actual typhoon. In contrast, the SNGL storm sustains the strength (930 hPa) during the simulation. The result indicates that the difference in the treatment of sea surface processes is considered to lead to be a crucial error (-20 hPa in this case) in a tropical cyclone intensity forecast.

In SNGL, the evaporation rate (over 1400 W/m2) is much larger than that in CPLD (600-800 W/m2), and shows more symmetric “ring” distribution around the eyewall. The sensible heat flux distributions also show the similar patterns, but the effect is secondary importance. The more intense surface total fluxes in SNGL are also consistent with the low-level warm core formation showing an erroneously high value of 385 K. The results suggest that excessively high evaporation (and heat transfer) rate is attributed to a strong positive feedback cycle and the positive typhoon intensity bias. In addition, it should be emphasized that the reasonable parameterization of the atmospheric-ocean-wave interaction is quite important for typhoon intensity forecast.

The rapid cooling of the sea surface (about -5 K) occurs at the east of the Chaba's center, where a strong southerly wind induces strong turbulent mixing. According to the ocean model output, the shallow mixed layer depths and strong stratification, which are favorable conditions for cooling of sea surface temperature, were promoted by the resultant vertical advection of cold water from below due to the Ekman pumping just beneath the slow moving typhoon. The results imply that in order to predict the realistic typhoon intensity, the full 3-D flow model, rather than a one-dimensional mixed layer model generally often used, must be applied to the atmospheric model.

The ocean coupling had an important effect on storm intensity. Inclusion of the ocean surface processes was found to be important to reduce the positive intensity bias during the forecast when the typhoon was greatly impacted by the reduced, reasonable supply of heat and moisture. It is expected that this coupled model can provide the early warning information not only about future position and intensity but also about the hazard of typhoon-related disasters (high wind, storm surge, and tidal wave) along the coastal areas.